Analysis of nitrous gases



March 1l, 1947. R. H. PARK ETAL vANALYSIS oF NIT-Rous GASES 2 Sheets-Sheet Il.

Filed Nov. l5, 1941 mi MW Taha NAX 4 E www, ,5

March l1, 1947. R. H. PARK ET AL 2,417,321

ANALYSIS oF NITRoUs GASES Filed Nov. l5, 1941 2 Sheets-Sheet- 2 QW www .wm Nb sm portions of both. So far Patented Mar. 11, 1947 Robert H. Park, Washington,

D. C., and James K.

Dixon, Riverside, Conn., assgnors to American Cyanamid Company, New

poration of lMaine York, N. Y., a cor- I Application November 15, 1941 Serial No. 419,244

This invention relates gases. More'particularly, itrelates to the det-erf mination` of lnitric oxide (NO), 'nitricperoxide (NO2), and mixtures-thereof, in gaseous atmospheres in which they may bevpresent.

`'7 Claims. (Cl. 23-232) to the analysis 'o'f nitrous In many chemical operations, such asin' the manufacture of nitric acid, nitrites, etc., it is important to know the nitricoxide and nitric peroxide content of the gases employed in the process or being wasted therefrom. Although chemical methods of analysis are available these meth- -ods are slow and by the time the results are known theoperating c onditionsof the process may have changed to such an extent that the analytical results obtained no longer-show' the conditions prevailing in the reaction zone. Y In such cases it is extremely diiiicult to control the process intelligently in accordance with scientific principles. The inherent disadvantages of the strictly chemical methods are avoided by the present invention which enables us to make the analysis rapidly andv accurately and, if desired, continuously-while recording the results automatically.

Our method of analyzing gaseous atmospheres containing NO and NO2,

ticularly upon light absorption changes which are caused to take place in the atmosphere byvirtue of changes in the chemical composition of the and mixtures thereof, is Adependent upon photometric processes, more pargases. 'Ihe light absorption values and changes i therein are measured photometrically and translated, by means to be presently described, Ainto analytical results.

The fact that NO2 is a yellowishred vapor under ordinary conditions has led to' the suggestion that the concentration of this gas in gaseous mixtures could be4 determined photometrically. Nitric oxide is a colorless gas, however, and its concentration in a mixture oaf-gases cannot be directly determined by photometric methods. As most industrial processes involving the use of these gases employ mixtures thereof, the relative proportions of which may iluctuate considerably,

it is important that the concentration of bothNO .to N205 which being colorless cannot be photofoxidatioi'i of N O to NO2 by it impractical to use this express the NO2 content of the gas as a function thereof. Since l\lG"-whichl may be present is readily oxidized, t'o NO2, the photometric measurements should be made fairly soon-after the Isample has vbeen taken. Also NO2 being readily absorbed in water with the lformation of HNOa and NO, lengthy contact with Water or water 'vapor shouldl be avoided.v

yTo determine the concentration of NO in the sample theNO contained ytherein is oxidized sub stanti'al'lycompletelyto NO2 and the light absorp. 'tionof the sample then measured photometrically,A The'differe'nce between the NO; content lcharacteristic lof the photometric value obtained 'for theorigin'al sample, and the corresponding "content of the sample after the NO has been oxidized to NO2 provides'a' measure 4of the NO con# ce'ntration of the original sample. f The'oxidation of NO to NO2 must be carried out lunder rather definite conditions,

Ordinary Oxy-- 'gen will oxidize NO- to NOzfbut the oxidation is very slow and'therefore cannot be relied upon for/oxidizing' purposes.' The time required for ordinary oxygen makes method in rapid analytical Wo'rkand makes it impossible to control fthe manufacturing process thereby with any degree of accuracy'.`

' We have found that by employing ozone, which .may be obtained by passing air through an ozonizer in known manner, it is possible to oxidize substantially all of the NO'to NO2 almost instantIy, and the photometric'measurement of the sample may be completed in a few seconds. The results 'thu'so'btained by our method'show the composition of the gases being sampled substantially at the moment of taking the sample.v

As ozone is a-Dowerful oxidizing agent an excess of Iozone will, after the NO has been oxidized to NO2, continue 'the oxidation of the NO2 in the gas J '-.metrically measured. We overcome this diiiiand NO2 be known since the eiiiciencyof the process may depend upon the quantitiesand pro- V as We are aware the rapid and accurate photometric determination of NO and mixtures of both NO and NO2 in gaseous atmospheres has not heretofore been? accomplished.

In accordance with the principles of our invention we determine the NO2 content of asample of the gaseous mixture by photometric measurement of the absorptionof light bythe sample and?- i -culty byconverting all the xed nitrogen in the .gasl sample intoNOz during the measurement of the-`light'absorption 'of the gas so that a true maximum color value is obtained whieh'expresses the .total oxides' of nitrogen'present in the gas.

Supply of the amount of ozonenecessary to .loxidize all of the NO 'to NO2 is l accomplished by us by variousrmethods. In one method which will :be described in greater detail in connection with Fig. 1 of the drawin r ume of the gases to be analyzed with a known g we mix a known volvolume of ozonized air containing'at least enough ozone to oxidize all of the NO in the sample to NO. The oxidation of .the N0 to NO2 'by the of the No to No2. air-gas mixture will agam be at a maximum but ozone takes place almost immediately. Should an excess of ozone Vbe present, however, part of the NO2 is furtheroxidized to N205. By passing th1s mixture of gases through a zone heated to temperatures of the order of 250-500" C. the N205 may be decomposed to NO2 and oxygen and the excess ozone decomposed to oxygen. At this point all of the NO originally present in the sample has been oxidized substantially completely to NOzian'd the gases may then be cooled to a standard temperature and the light transmission measured photometrically and expressed in terms of its nitrogen peroxide content. When ymeasuring gases substantially free from moisture, or when moisture in the gas has been removed by a suitable dehydrating agent, the `light absorption of the ozonized mixture will remain substantially constant and can be measured by the operator by means of the apparatus described in connection `with Fig. -1.

Another method of supplying just enough ozone v.to oxidize all of the NO in the sample to NO2 is de-Y scribed in greater detail in conjunction with Fig.

2. By this method a mixture of air and the gases to be analyzed,v in measured proportions, is passed through an absorption cell and the light transmission of the sample measured and recorded continuously. This measurement may be made ,by proper standardization to express the NO2 content of theA gaseous mixture. An electric poten- .tial is then applied to the ozonizing apparatus vand it commences to supply ozone to the air Ystreaml in steadily increasing amounts. As the quite small Vthere will not. be enough ozone in the air-gas mixture to oxidize all of the NO to NO2. As the amount of ozone increases the percentage' of NO being oxidized to NO2 also increases and the color of the air-gas mixture deepens.- At that point at which there is just enough ozone being supplied to oxidize all of the NO to NO2 the air-gas mixture will be at its deepest color.- As an Vexcess of ozone is supplied by the Aozonizer the excess will be available for oxidizing ANO2 to N205 and the color of the air-gas mixture will become lighter. As the photometric values are recorded continuously while the amount of ozone in the gas increases they will be seen to approach a point of maximum light absorption and then fall 0E. vThevmaximum point on a curve plotted from these values will represent the light transmission of the sample when all of the NO has been oxidized to NO2, or, in other words, the point where just enough ozone has been added to oxidize all of the NO to NO2. From values previously determined by standardization of the ap- .paratus the maximum value thus obtained may be made to express the total NO2 content ythen in the gas. As the NO2 content of the gas before being subjected to oxidation had been determineda few seconds previously it is a comparatively simple matter to. calculate the amount of NO2 developed in the mixture by oxidation vand from Athis theamount of NO originally present in the gas sample.

When an excess of. ozone has been generated the ozonizer may then be thrown out of operation. `'The' steadily decreasing amount of ozone in the gaseous mixture again reaches a pointat which there is just enough ozone to oxidizegall At this point the color ofthe again falls away as the amount of ozone becomes Our invention will now be illustrated'in greater detail by means of the drawings, Fig. l of which illustrates diagrammatically suitable apparatus for carrying out one modication of our invention and Fig. 2 represents diagrammatically another type of apparatus for applying the principlesrof our invention in a diierent manner. It Will be understood, however, that our invention in its broader aspects is not limited to the particular modifications shown since obvious changes therein may be made without departing from the scope of the invention set forth in the appended claims.

Referring now to Fig. 1 a sample of they gaseous atmosphere to beanalyzed for NO, NO2 or both NO and NO2 is taken through conduit I to a filter 2 which may be packed with glas-s wool or some other non-reactive material effective to lter dust and other foreign objects from the gases. The iilter may also contain dehydrating agents such las anhydrous calcium sulfate or phosphoric acid to remove excess moisture which may under certain conditions be present Vin the gases. The gases are thenV led from the filter through a conduit 3, iiow meter 4, valve 5, conduits E, l and 8 into an absorption cell 9. The various conduits and valves may be constructed of glass, stainless ysteel or some other material resistant to corrosion by nitric acid. The absorption cell consist-s essen,- tially of a glass chamber, having plane glassends transparent to light. through the system by means of a stainless steel steam ejector, pump or by other suitable means (not shown) attached to conduit I0. Y

When the gaseous mixture drawn into the absorption cel contains NO2 it will have a pale yellow 'to yreddish-yellow color, the depth of which depends upon the concentration of NO2 present. To measure the concentration of NO2 gas in the atmosphere the absorption of light passing through the cell due to the presence of the NO2 in the gas is measured. This is accomplished by Ameans of the photometric circuit shown in which light from a suitable source, such as a lamp I I of about 6 oandlepower intensity, passes through a Alens I3 and onto the sensitive face of a matched photoelectric cell I4. The light absorption is then measured by means of a galvanometer I5 which forms a part of the circuit shown. This circuit comprises a sensitive galvanometer, the particular one shown having Van internal resistance of 1100 ohms, a sensitivity of 0.008 microamp per millimeter and having a critical damping external resistance of 6200 ohms. Thev circuit also contains two variable resistances I6 and I1 of 400 ohms each and xed resistances I8, I9, 20 and 2l as shown. The circuit also has a switch 22 with points 23 and 24. l

Before measuring the light absorption of the' gases in the cell the galvanometercircuit is bal- The gases are drawn anced by placing switch v22 on contact point 24 and while the cell contains nothing but clear air the galvanometer is balanced by the variable resistance I6. With NO2 gas in the absorption cell the switch 22 is placed on contact point 23 and the galvanometer again brought to balance by the variable contact switch 25 by sliding its contact point along the resistance I1. By means of a previou-s calibration of the apparatus which has been accomplished by placing gases having known quantities of NO2 in the absorption cell and balancing the circuit by the variable switch 25 one may, by knowing the position of the switch 25 when an unknown gas is in the absorption cell, determine from these previously found values the NO2 content of the unknown gaseous atmosphere.

Having determined the NO2 content of the gases under analysis the operator may then determine the NO content of the gas by oxidizing the NO to NO?l and again determining the NO2 content then in the gaseous mixture. To do this a stream of air is passed through a filter 30 similar in construction to the filter previously described, thenl througha now meter 3i, valve 32 and into an ozonizer 33. The particular ozonizer shown consists essentially of two glass tubes of dilerent diameters having walls about l mm. thick and about 48 inches long. The smaller diameter tube lits inside the larger leaving a space between the two glass walls of about 2 mm. The inside of the inner tube and the outside of the outer tube are covered with tin foil or some other electrical conducting means. An electric potential of 5,00010.000 volts applied across the space between the twotubes causes the formation of ozone when oxygen is passed through the apparatus. As the air is drawn between the walls of the ozonizer 34 a portion of the oxygen therein is converted to ozone. The ozonized air then passes into conduit 3'! through valve 36 and into conduit 6. The ozonized air and the nitric oxidecontaining ga-s under analysis are caused to mix in line 6 in proportions which may be determined from the rates of flow as shown by the iiow meters. The mixed gases then pass into a reaction chamber 4D. The purpose of the reaction chamber is to allow complete oxidation of NO to NO2 bythe ozone. Experience. however, has shown us that because of the rapidity ol the oxidation the reaction space may be very small and in fact if the conduit is of an appreciable length no reaction chamber is necessary,

lf just enough ozone is present in the gaseous mixture in the chamber 40 to oxidize all of the NO to NO2 the light absorption of the gaseous mixture may be measured at once. However, it is very diiiicult to control the amount of ozone generated by the ozonizer and an excess is ordinarily employed. The excess ozone as explained above oxidizes part of the NO2 to N205 which is colorless. The gases are therefore passed through a furnace 42`where 'they are heated to temperatures between Z50-500 F., 350 F. being the preferred temperature. The furnace consists essentially of nothing more than a heat resistant covering for conduit 1, such as an Alundum tube, wrapped with Nichrome resistance wire and the assembly covered with suitable insulating material. At the high temperatures of the furnace the N205 which may have formed is broken down to NO2 and oxygen, and excess ozone is broken down to oxygen.

Although an excess of ozone has been mixed with the nitric oxide containing gases it will be 'seen that as the gases emerge from the furnace 6 they will be in the same chemical state as though all of the NO has been oxidized with just enough ozone to oxidize it to NO2. The hot gases are then cooled to substantially room temperature in the cooler 43 and then led by conduit 8 into the absorption cell. The NO2 determination is again made as previously described. Since the increase in NO2 content represents the amount of NO in the sampled gases it is a very simple matter to calculate the NO content of the gases.

Although we have described an apparatus for carrying out this modification of our process in some detail it will be understood of course that other photometric circuits may be employed. The photoelectric cells may be of either the vacuum tube type or the barrier layer type. We have shown a circuit employing two photoelectric cells but'this is merely for the purpose of compensating for fluctuations in the. light source and if a light of dependable uniform intensity is made available a single photoelectric cell could be employed with suitable changes in the circuit. Since photoelectric measurements of this type .are well known further elaboration appears to be unnecessary. The comprehensive'article Photoelectric methods in analytical chemistry, Ralph H. Muller, Industrial & Engineering Chemistry, analytical edition, vol. 11, pages 1-17 (1939), for example, describes several suitable photometric circuits in detail.

Fig. 2 shows a method of analyzing NO and NO2 containing gases automatically and continuously while at the same time recording the results. In this modification of our process we may employ a photometer of the flickering beam type, such as described in detail in the patent to Orrin W. Pineo, No. 2,107,836, dated February 8, 1938.

In this apparatus light from a suitable source 5l! passes through a system of lenses 5l. and a prism 52. A movable slit formed by a mirror 53 and knife edge 54 allows a substantially monochromatic beam of light to be reilected from the mirror .surface through additional lenses 55 and prism 56. When using this apparatus we disconnect the knife edge and mirror assembly from the balance 0f the recording device as described in the Pineo patent and maintain it at a position where blue light is selected since we are not interested in varying the chromatic quality of the light beam as the recorder operates. The monochromatic beam is then caused to pass through a slit 51, a Rochon prism 58, a Wollaston prism 59 and a fllickering prism 60 mounted in the armature 6! of a synchronous motor. The resulting two beams which have been polarized at right angles to each other by the optical system pass through lenses 62. One beam passes through a reference cell 63 and the other passes through an absorption cell 64 such as previously described. The two light beams then enter an integrating sphere 65, containing a photoelectric cell 66. In a photometer of the ickering beam type the absorption of the light passing through the absorption cell is measured by the angle through which the Rochon prism 5B is turned. The angle through which the Rochon prism is turned is recorded on coordinated paper wrapped on a recording drum 5'! by a stylus (not shown) connected by suitable means 68 to the Rochon control mechanism.

VariousA mechanical modifications in thisphotometer may obviously be made without departing from the spirit of our invention. For example, a light'with a suitable collimating lens,'slitvand blue filter may be mounted infront of the photometeringY prism thereby avoiding the-necessity of using the complex optical system described up to that point. Also, a continuous strip chart may be employed in place of a single sheet chart. Various types of cams controlling the relation between prism and pen motion may also be employed but since these have beenv previously described it does not appear to be necessary to describe them in detail here.

Another useful modication which enables us to record the photometric data at a point removed from the instrument comprises a rheostat coupled to the photometering prism and forming a part of a Wheatstone bridge circuit. This circuit may be used to control the pen of a standard electrically operated recording device at some distant point as, for example, the control laboratory.

The analysis of nitric oxide containing gases in this type f apparatus will now be described. A

sample of gas to be analyzed for NO and NO2 .is

drawn through a iilter and iiow meter 1l similar to those described in conjunction with Fig. l. A stream of air drawn through filter 12, iiow meter 13,V conduit 14, valve 15, ozonizer 16 and valve 11 is led into mixing chamber 19. This chamber allows us to add progressively increasing amounts of ozone to the air stream which is mixed with the nitric oxide containing gases in conduit 8G. The volume of chamber 19 is large relative to the volume of the absorption cell 64.

At the startofthe analytical cycle'the ozonizer is not in operation 'and no ozone is mixed'with the nitric oxide gases. The nitric oxide-air mixture in line 8B is drawn through absorption cell 64 by a pump or other means (not shown) attached to line 8i. The absorption of the light passing through the absorption cell is measured by the angle at which the Rochon prism is automatically turned by the control motor 82 yand is recorded by the stylus on the coordinate paper. lA synchronous motor 83 turns the recordingfdrum at a known constant speed. By simply glancing at the line traced on the graph paper by -the stylus the operator is able to determine the relative concentration of the NO2 in the gases passing through the absorption cell. The proportion of air mixed with the gases being analyzed must of course be reckoned with in the analysis but this is readily determined by l consulting the flow meters which may be constructed so as to pass constant flows of gases in any desired proportion.

The concentrationof NO2 in the gaseous atmosphere having been determined, a time cycle controller 84 operating at predetermined periods of time allows a high electrical potential to be applied across the ozonizer and results in the generation of ozone in the air-gas mixture. As the amount of ozone in the air-gas mixture increases more and more of the the NO is oxidized to NO2 and the color of the gaseous mixture in the absorption cell becomes darker and darker. At a point at which just enough ozone has been added to the mixture to oxidize all of the NO to NO2 the color of the gases in the absorption-cell will be ata maximum and the recording stylus will Vregister onA the coordinate paper this maximum value. As the amount of ozone increases further some of the NO2 will be oxidized to N205 and the color of the gaseswill againdecrease and may, with the generation of large quantities of ozone, become practically colorless. The recording stylus will in such case register practically no light absorption. The time cyclecontroller then operates to cut offthe supply of electric current to the ozonizer and the amount 1 lessand less NO2 is oxidized 'of NO contained in the gas.

of ozone inthe air-gas-mixture decreases.

to N205 in the moving gas stream the color of the gases in the absorption cell will again become darker and again at a point at which there is `just enough ozone to oxidize all of the NO to NO2 absorption of light in the absorption cell will be at a maximum and Iwill be recorded bythe stylus on the coordinate paper. As the production of ozone decreases to a value of zero the color of the gases being drawn through the absorption cell will again correspond to the color of the gases being sampled and the recording stylus will register values corresponding to gases. By observing the maximum color change as recorded on the coordinate paper it is a simple matter to determine the NO content of the gases from previous calibrations of the apparatus. The maximum color absorption recorded will represent Vof course the total NO2 in the absorption cell due to the NO2 originally present in the sampled gases plus the NO2 formed by oxidation A The above described cycle may then be repeated continuously or at intervals throughout any desired time'and the recorded results may be preserved for future reference. Y

In this method of determining the NO and NO2 content of the gaseous atmosphere it Will be seen that as in the first described modification the NO content is determined by mixing withV the gases to be analyzed a quantity of ozone 'sufiicient to oxidize all of the NO in the gases to N92 and measuring photometrically a maximum color change of the gases asa function of the NO2 content thereof.

VAs will be apparent from the foregoing'our method oi analyzing rgaseous atmospheres for NO,`NO2, or both, may be applied under a wide variety of conditions. Our method is of greatest value, however, in chemical processesV in which gases containing NO and NO2 in varying proportions are employed. We may k:for example Y withdraw samples. of gases in the various levels between the absorption plates of .a nitric acid absorption system and control the absorption process more eihciently from the results-of the.,

analysis. VBy regulating the ammonia burners We may controltthe relative proportion of NO and NO2 in any part of the absorption tower. The NOrNOz ratio may also be changed by decreasing the period of Contact of the gases in the plant, by decrease in pressure or even by changing the design of the nitric acid absorption tower. We may also determine the NOrNOz ratio in the gases issuing from the nitric acid plant and may modify the ratio as desired to obtain a more eflicient operation of either the nitric acid plant or of the nitrite plant which is generally associated Iwith it. We may also employ our method in the analysis or the gases in various parts of the nitriteplant or employ itto determine the gases-issuing to waste from the I plant. By having a rapid analytical method of determining the NO and NO2 content of the gases from these plants their efficiency may obviously be improved to an extentnot heretofore obtainable; y f

vWhat we claim is:

l. A method of determining the NO content' oi"v a gaseous atmosphere which comprisesA con-` verting the NO in said gaseous atmosphere sub'- stantially completely to NO2, and measuring photometrically the resulting maximum. change in a light absorption characteristic of said .gas-

the NO2 content of the eous atmosphere as a function of the NO content thereof.

2. A method of determining the NO and NO2 content of a gaseous atmosphere which comprises measuring photometrically a light absorption characteristic of said atmosphere and expressing the content of NO2 in said atmosphere as a function thereof, then oxidizing the NO in said atmosphere substantially completely to NO2 and measuring photometrically the resulting maximum change in said light absorption characteristic of said atmosphere as a function of the NO content thereof.

3. A method of determining the NO- content of a gaseous atmosphere which comprises progressively mixing with said atmosphere a quantity of ozone sufficient to oxidize all of the NO in the atmosphere to NO2 and measuring photometrically the maximum change in a light absorption characteristic of said atmosphere during the progress of the mixing process as a function of the NO content thereof.

4. A method of determining the NO and NO2 content of a gaseous atmosphere which comprises measuring photometrically a light absorption characteristic of said atmosphere and eX- pressing the content of NO2 in said atmosphere as a function thereof, then progressively mixing with said atmosphere a quantity of ozone suicient to oxidize all of the NO in the atmosphere to NO2 and measuring photometrically the maximum change in said light absorption characteristic of said atmosphere during the progress of the mixing process as a function of the NO content thereof.

5. A method of determining the NO content of a gaseous atmosphere which comprises mixing with said atmosphere a quantity of ozone at least suiiicient to oxidize all of the NO in the atmos phere to NO2, heating the gases to temperatures sufficiently high to reduce any N205 which may have formed to NO2 and to destroy excess ozone, and then measuring photometrically the change in a light absorption characteristic of said atmos- L.

phere as a function of the NO content thereof.

6. A method of determining the NO and NO2 content of a gaseous atmosphere which comprises measuring photometrically a light absorption characteristic of said atmosphere and expressing the content of NO2 in said atmosphere as a function thereof, then mixing with said atmosphere a quantity of ozone suiiicient to oxidize all of the NO in the atmosphere to NO2, heating the gases to temperature sufliciently high to reduce any N205 which may have formed to NO2 and to destroy excess ozone, and then measuring photometrically the change in said light absorption characteristic of said atmospherev as a function of the NO content thereof.

7. A method of determining the NO and NO2 content of a gaseous atmosphere which comprises the steps of mixing a stream of air and a stream of the gaseous atmosphere containing NO and NO2 and measuring photometrically a light absorption characteristic of said atmosphere and expressing the content of NO2 in the mixed gases as a function thereof, then forming a progressively increasing quantity of ozone in said air stream in amounts suicient to oxidize all of the NO in the mixed gaseous atmosphere to N01; while measuring photometrically the light absorption characteristics of the mixed gases and determining the maximum change in the light absorption characteristics as a function of the NO content thereof.

ROBERT I-I. PARK. JAMES K. DIXON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,409,912 Reid Dec. 20, 1921 1,919,858 Pettingill s July 25, 1933 1,977,359 Styer Oct. 16, 1934 OTHER. REFERENCES 

